It appears to be a solid lifter- - - -a hydraulic would have a spring clip holding the movable plunger into the top of the main body and it would slowly compress if a steady pressure was put on the pushrod seat for a couple of minutes with oil seeping out of the lifter as the plunger drops. Turn the engine until one of the lifters drops to its lowest position and measure the distance from the pushrod seat to the adjusting bolt ball and you'll be within 1/8" or less of the necessary pushrod length for the combination of parts you have. The travel of the adjuster thread will give you a bit of leeway on the length.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!